Ventricular tachyarrhythmias and atrial fibrillation are the most important arrhythmias affecting patients. They are the most frequently encountered tachycardias, account for the most morbidity and mortality, and despite much progress, remain therapeutic challenges. Invasive electrical studies of the heart (electrophysiologic studies) are often used in the diagnosis and therapy of arrhythmias, and may arrhythmias can be cured by selective destruction of critical electrical pathways with radiofrequency (RF) catheter ablation. A major limitation in studying arrhythmias in patients, however, is the lack of ability to accurately correlate anatomical and electrical information. Anatomy is derived from x-ray images, which are two-dimensional and have substantial anatomic ambiguity. Another major limitation is the lack of ability to visualize ablated areas of myocardium during catheter ablation procedures, making it difficult to confirm the presence of ablated lesions in the desired locations. We have developed ways of combining the anatomic information from magnetic resonance imaging (MRI), with electrophysiologic testing and catheter ablation. We hypothesize that magnetic resonance imaging, with transesophageal receivers, intracardiac receivers and MRI- compatible (non-magnetic) electrode catheters, can (1) provide accurate navigation of catheters without radiation, (2) provide the ability to visualize ablated lesions, and (3) aid in producing more accurate electrical maps. As a prototype for the development of new approaches to electrophysiologic testing and catheter ablation, this proposal addresses atrial fibrillation primarily. The imaging technologies developed in this project, should however, be broadly applicable to using MRI to guide interventional procedures in the heart in general, as well as in other organ systems. This project is an ongoing partnership between the Johns Hopkins University School of Medicine, the Johns Hopkins University Applied Physics Laboratory, Surgivision Inc., Robin Medical Inc., and Bard Electrophysiology, Inc., all of which have supplied resources to the project, and will continue to cost share. The School of Medicine is the lead institution and will be (1) developing specifications for advanced imaging and ablation catheter systems, (2) testing new technology as developed, and (3) performing interventional studies. The Applied Physics Laboratory is developing the technology for advanced intracardiac and transesophageal MRI receivers, and is developing software for 3-dimensional reconstruction of MR imaging. Surgivision is developing clinical-grade versions of the MR receivers. Robin Medical is developing technologies for precisely localizing the tip of a catheter inside an MRI scanner. Bard Electrophysiology is supplying non-magnetic electrode catheters for use in the MRI scanner.